1. Field of the Invention
The present invention relates to a thermal transfer sheet comprising a substrate sheet, a dye layer, a white layer(white color layer) and optionally a receptor layer, alternately disposed side by side on a surface of the substrate sheet. Further, the present invention relates to a method for manufacturing the thermal transfer sheet.
2. Description of the Related Art
A printed product, which comprises a transparent transfer receiving material and an image formed on one side thereof so as to allow the image to be identified or enjoyed from the other side of the transparent transfer receiving material, is conventionally used in, for example, an electric decorating display for advertisement or decoration purposes. Usually, this electric decorating display comprises a transparent transfer receiving material/image/white layer, stacked in this order, so that the display is enjoyed by irradiating it either from the front side (the side of transparent transfer receiving material) or the back side (the side of white layer). In addition, the image formed by transfer can be made more distinct by appropriately transmitting or diffusing the irradiating light. Usually, for the purpose of diffusing the light, an electric decorating device, or a light diffusing layer is attached to the electric decorating display.
Heretofore, the electric decorating display is manufactured by a process comprising the steps of forming an image or letter by means of, for example, offset printing or gravure printing on a transparent plastic sheet which constitutes a transfer receiving material and then forming a solid printed with a white ink on the image or letter. This process for manufacturing an electric decorating display is suitable for the mass production of one and the same printed product.
Recently, however, in order to meet demands for, for example, personal use and many kinds in small quantities, it has become necessary to change letter or image every each sheet or small number of sheets.
In order to meet these demands, a desired printed product is manufactured by a process comprising the steps of transferring a receptor layer, which facilitates the image formation, from a receptor layer-transferring sheet to a transparent transfer receiving material such as a transparent plastic sheet, forming an image by ink jet printing or sublimation thermal transfer printing on the receptor layer, and transferring a white layer from a white layer transfer sheet onto the image thus formed. The sublimation thermal transfer printing in particular is drawing attentions because this method is expected to provide excellent continuous gradation and a full-color image comparable to that of a color photograph.
A sublimation thermal transfer process for producing a printed product required at least the transfer of a dye layer and a white layer, and further required the transfer of a receptor layer prior to the transfer of the dye layer if a transfer receiving material had insufficient dyeability. Accordingly, the sublimation thermal transfer process required the use of a plurality of thermal transfer sheets. In order to simplify the process, Japanese Patent Application Publication (JP-B) No. 7-77832 discloses an integrated thermal transfer sheet comprising a substrate sheet, a receptor layer, a dye layer and a white layer, wherein these layers are alternately disposed side by side on a surface of the substrate sheet as well as a method which enables a printed product to be manufactured at a lower cost by successively transferring these layers from the sheet to a transfer receiving material. The integrated thermal transfer sheet disclosed comprises a substrate sheet, a receptor layer-transferring portion having a three-layered structure composed of a peeling layer/a receptor layer/an adhesive layer, a dye layer for colors, and a white screen(white background)-transferring portion having a three-layered structure composed of a peeling layer/a white ink layer/an adhesive layer, alternately formed and disposed side by side on the substrate.
The above-mentioned integrated thermal transfer sheet, however, is associated with a problem that, since the process for forming the layers requires a number of steps including coating and drying, the process is complicated if the number of the layers is large, and, as a result, the production cost of the thermal transfer sheet becomes higher.
Another problem is that the white screenability of the printed product obtained by the above-mentioned process is poor, and this problem of insufficient white screenability of the printed product is associated with conventional processes.
In addition, it has been demanded to improve the conventional level of releasability of a receptor layer from a thermal transfer sheet supporting the receptor layer when the receptor layer excellent in dyeability is transferred to a transfer receiving material which has poor dyeability.